In the integrated-circuit (I.C.) chip packaging industry, photo-imageable materials, such as photoresists, photo-imageable polyimide, and other photo-imageable dielectrics, are used to form patterns on substrates, such as for example semiconductor wafers, and on the various material layers that may be disposed on the substrates. Of interest to this disclosure are the well known trench and via patterns, which are voids formed in a material layer (e.g., photoresist) disposed on the substrate. The material within the trench and via structures is removed, and often replaced by a different material. FIG. 1 shows several examples of trench structures formed in a material layer 11, which is disposed on a substrate 10. Various trench structures are shown at 20, 30, 40, and 50. Various via structures are shown at 60 (cylindrical) and 70 (square-shaped). As is known in the art, via and trench structures may be combined to form other geometrical structures. For example, the well known "dog-bone" structure is shown at 80 in FIG. 1.
A typical application of a via (or trench) structure may be found in the construction of an electrical coupling between two parallel metal layers which are separated by an insulating dielectric material. A first metal layer is initially formed on the substrate, followed by the formation of the insulating material. Vias are then formed in the insulating layer. If the layer is photo-sensitive, the vias may be formed by patterned exposure to actinic radiation followed by dissolving in a developer solution. Alternatively, the material with the vias may be removed by a wet etch or dry etch process (e.g., plasma or reactive-ion etching) through an etch mask. Next in this typical application, a second metal layer is disposed over the insulating layer, filling the voids of the vias (or trenches) and making contact to the first metal layer. Alternatively, metal may first be formed within the vias before the deposition of the second metal layer. In many applications, the insulating material is left on the substrate. In some applications, the insulating material is removed (as for example as done in forming "air-bridges").
The present invention is directed to forming trench and via structures in photo-imageable materials, such as for example photoresists. Photoresists, as well as all photo-image, able materials, are generally classified into two groups: positive type materials and negative type materials. Positive-type materials become soluble in a developer solution (e.g., alkaline-aqueous solution) when exposed to light radiation, usually ultraviolet. Negative-type materials become insoluble in a developer solution (e.g., an organic solvent) when exposed to light radiation. In either case, the light generally must be within a given range of wavelengths, the range being a function of the chemical and optical properties of the particular photo-imageable material. Light within the wavelength range is often referred to as "actinic" radiation. As used herein, actinic radiation for a given material is any electromagnetic radiation having a wavelength capable of activating that material.
A typical negative photoresist comprises a synthetic polymer resin (e.g., cyclized polyiosoprene rubber), a photo-initiator chemical (e.g., azide compounds), and a solvent base. The solvent base dissolves the resin and initiator chemical, which are usually solid or near solid. This enables the photoresist to readily coat a desired substrate (e.g., semiconductor wafer, ceramic substrate) to form a photoresist layer thereon. After coating, the solvent is evaporated from the substrate by exposure to moderate heat ("soft bake"), leaving the resin and photo-initiator as a hard layer.
The resin of a negative-type photoresist generally comprises polymer chains, each chain having one or more unsaturated carbon bonds. With sufficient energy, unsaturated bonds from two such chains may be bonded to form a saturated carbon bond. This cross links the chains and, in combination with other cross-links to other chains, renders the photoresist substantially insoluble in a selected organic solvent. The energy is generally provided by way of the photo-initiator chemical. Upon exposure to actinic radiation, the molecules of the photo-initiator absorb energy from the radiation and interact with chains to form cross links. Some resins are capable of cross-linking without the aid of a photo-initiator and are directly responsive to the actinic radiation.
A typical positive photoresist comprises a resin, a photo-sensitizer chemical, and a solvent base. As before, the solvent base dissolves the resin and sensitizer chemical, and is evaporated after the wafer has been coated to form a layer thereon. The resin's polymer chains are normally insoluble in alkaline-aqueous solutions. Upon exposure to actinic radiation, the molecules of the photo-sensitizer decompose into acidic products which, in the present of an alkaline-aqueous solution, promote the dissolving of the polymer chains. The exposed portions of the positive-type photoresist then become soluble in an alkaline aqueous solution.
In use, a typical photoresist (either positive or negative type) is coated on the wafer to form a photoresist layer, soft-baked to remove the solvent base from the layer, and then exposed to actinic radiation through a mask. The mask transfers its pattern to the photoresist layer. The photoresist layer is then developed by exposure to an appropriate developer solution. For a negative photoresist, the unexposed portions of photoresist dissolve in the developer solution. For a positive photoresist, the exposed portions dissolve. A pattern is then left on the wafer for further processing. Once the remaining photoresist layer is no longer needed, it may be removed by exposure to an organic solvent called a stripper.
Within approximately the last five years, a few positive-type photoresists have been developed which can reverse their exposed images. These resists are called Image Reversal (or Reversing) Photoresists (IRP's), or dual tone resists. They are classified as positive resists because their chemical structure is closer to that of positive photoresists rather than that of negative photoresists. In an unexposed state, the IRP materials are insoluble in alkaline aqueous solutions. After exposing an IRP material to actinic radiation though a mask, the exposed portions become soluble in an alkaline aqueous solution. The unexposed portions remain insoluble. The IRP material may then be developed as a positive resist. In the alternative, the resist may be heated to a temperature of around 100.degree. C.-160.degree. C. to render the exposed portions insoluble. By then exposing the previously unexposed portions to the actinic radiation, the previously unexposed portions are rendered soluble in the alkaline aqueous solution. Accordingly, the initial mask image is reversed, and may be developed to form a negative image of the mask.
In this reversal process, the second exposure to actinic radiation may expose the entire layer rather than just the previously unexposed portions. The type of exposure is called a "blank flood exposure" in the processing art. Due to the heat treatment, the initially exposed portions remain insoluble, even if exposed to the actinic radiation in the second exposure.
At present, photoresist and photo-imageable materials, including IRP materials, are generally limited in their ability to define high-aspect ratio via structures. Due to optical diffraction effects which occur during the exposure of actinic radiation through the mask, the aspect ratio in typical photo-sensitive materials is generally limited to a maximum of 2:1 (height:width). Additionally, it is difficult to control the dimensions at the bottom of a patterned structure in photo-sensitive materials when the aspect ratio is 2:1 or more. The control of the bottom dimension is critical in many applications because such vias are intended to contact small features on the layers or substrates below.
Some advanced photo-imaging processing materials and techniques, such as contrast enhancement materials (CEMs), can achieve aspect ratios higher than 2:1, but are generally limited to relatively moderate heights. For CEMs, the aspect ratio that can be achieved is inversely related to the height that can be achieved. Typical CEMs can achieve a 3:1 aspect ratio at a maximum height of 25 .mu.m to 30 .mu.m, with the maximum height decreasing as the desired aspect ratio increases. Dry etching methods using multi-layer resist/mask structures can achieve aspect ratios greater than 2:1, but they are expensive and relatively complicated, and it is often difficult to control etch dimensions and uniformity with these methods. Additionally, these dry etching methods have not yet been adequately developed or proven for etch depths greater than approximately 10 .mu.m. Dry etching mask materials having higher etch selectivity than those currently available will be required for depths greater than 10 .mu.m.
Accordingly, there is a need for a simple and economical method for forming tall structures, such as vias and trenches, having high-aspect ratios with photoresist and photo-sensitive materials. There is also a need for a simple and economical method of controlling the bottom dimensions of high aspect-ratio structures.